1. Field of the Disclosure
The present application relates generally to optical systems for sensing state of focus of an image or a focused beam and, more specifically, to an optical apparatus and autofocus system which compensates for stray or reflected light from its own light source.
2. Description of the Related Art
A microscope is an optical instrument that has historically employed two or more lenses to make enlarged images of minute objects. In a so-called compound microscope, an objective lens is placed near a target to be viewed and the user views an image of the target through an eyepiece lens. In order to provide a clear and accurate image of the target, the microscope must be properly focused. Typically, microscopes can be focused either manually by the user or by built-in focusing system.
Focusing systems can be either passive, that is acting upon the quality of the collected image to find the best focus; or they can be active, that is, probing the target to find the correct distance for best focus. Occasionally sound or even jets of air have been used to probe for best focus, but modern actives systems typically use optical means.
An active optical focusing sensing system is an optical device that senses the state of focus of, for example, an image-forming device or a light-focusing device such as a microscope, by emitting one or more beams of light that are made to reflect from the surface under examination. The reflected light is then collected and processed so that the state of focus of the optical system may be controlled. In the case where the focus of the optical system is controlled dynamically it may be used as a component of a focus controlling system, that is, an autofocus system.
Prior art devices include optical systems, in which assemblies of light emitters, lenses, mirrors, filter, windows, detectors and similar optical components are combined so that light may be used as a measurement tool. Some of these devices measure qualities of light that have been reflected from a surface under investigation to determine properties of the surface, an optical system, or both. Many prior art devices project a spot of light through an axially symmetrical objective lens onto a target surface. Other focus sensing devices have used one or more toric lenses to form a line focus which is projected onto a target surface.
In general, a toric lens is one which has different curvatures in two principle sections. A cylinder lens is a limiting case of a toric lens where the radius of curvature in one section is infinite, that is, the section is flat, while the radius of curvature in the other section is finite, but non-zero. Toric lenses introduce astigmatism into the outgoing beam of light. Instead of having a single point focus, an astigmatic system has two line foci. Where astigmatic lenses have been used previously by others, only one of the two astigmatic line foci has been used. In those applications the line focus has been used to perform an averaging function to reduce the effect of local asperities in the surface under observation.
In another example, a light beam is projected onto a microscope sample in order to determine whether the sample is correctly focused. An error signal derived in this way can be used to maintain correct focus of the microscope in order to stabilize the image or to remain focused upon moving surfaces. Such an auxiliary optical system—an autofocus system—may be used to automate an inspection system that includes a microscope. The general result is improved image quality or improved irradiance control at the target because of better response to changing focus and stability of focus over time than could be provided by a human observer. Focusing times of a few milliseconds and axial position sensitivity of a fraction of the wavelength of light can be achieved by these autofocus systems. Typically active focus sensing systems are used in applications where greater speed is required, or where the surface under study is extremely uniform and there is essentially no detail for a passive focus sensing technique to use for measuring the state of focus. Many other types of optical systems have been used in prior art to make measurements or to study small objects and will not be mentioned here because they are similar to those of autofocus systems.
Idealized optical systems focus light, produce images or make measurements, subject to theoretical limits that apply to optical and measuring devices. These limiting factors include diffraction, optical aberrations, electronic noise, pixel dimensions in the case of pixelated cameras, dynamic range and sensitivity, sampling rates, etc. Among other things, theoretical limits describe the smallest resolvable features, the least amount of light detectable and the largest axial or radial region in which the image is sensibly well focused and sharply defined, assuming a system that is free of manufacturing errors and aberrations. An autofocus system has the task of maintaining correct focus to help deliver the best possible optical system performance.
For example, lenses are refractive devices. That is, they depend on discontinuities (or sometimes continuous internal variations) in the refractive index of light through a transparent medium in order to bend light as it transits the optical component. In this way they can form images or selectively sample a beam of light. However, even idealized lenses reflect light from their surfaces, where there is a discontinuity in refractive index. The resulting reflected light usually decreases the performance of an optical instrument, because the resulting so-called stray light decreases image contrast or introduces errors into optical measurements. Single or multilayer thin coatings of dielectric materials can be used to reduce, but not completely eliminate these reflections, especially when light of varying wavelength (color) transits the system. Other idealized limitations on optical performance can substantially affect achievable optical performance, but an optimally focused optical system will always produce better results than a poorly focused one.
In the case of an active focus measurement or controlling device, stray light can cause problems, especially when the target is poorly illuminated or reflects little light. An ideal focus measurement/control device must correct for variation in the reflection at a target surface either by changing the output power of the source, by changing the gain of the receiving circuit or by accepting a loss of gain in the focus signal generating part of the device. When the device is initially constructed a correction for stray light is often included by using a target surface whose reflectance is fixed, so that the optical system is clearly focused. However, in practice the optical system must respond to targets of varied reflectance, and the original stray light correction may no longer be appropriate. In this case the result of stray light is an error in measured focus that varies with the amount of reflected light. At one output level there is zero error, but at other output levels there is a measurable error. What is needed then is an autofocus system that compensates for the effects of stray light.